A single misfiring spark plug will affect the gas mileage much more than just one mile per gallon. A four-cylinder engine with a misfire stands to lose 25% of available power. At highway speed, spark plugs typically fire 18 times each second, that’s 64,800 times each hour. If you own one of the many vehicles on the road with distributorless ignition, that number changes to 129,600 times each hour the spark plugs fire. We recommend a tune up every 30,000 miles or less. During a tune up the spark plugs are replaced, the ignition timing is checked and set to specification, carbon is cleaned from the throttle plates and idle speed control system, the filters are checked, the pcv valve is replaced, and the ignition and fuel systems are inspected for worn or malfunctioning parts. All parts that are at or near the end of their service life should be replaced at this time.

What if my manual doesn’t require a tune up for 100,000 miles? This decision is ultimately yours, but here is my opinion. Manufacturers that recommend a long tune up interval are speaking of spark plug replacement only. The original equipment spark plugs are usually high quality double platinum plugs that wear much slower than conventional ones do. This is good news. On the down side, a spark plug that is not removed from the engine for such a long period of time may be very difficult or impossible to remove when it comes time for replacement. The other services included with a tune up are also neglected. Things like pcv valve replacement, timing check, throttle bore cleaning, fuel and air filter replacement, ignition wire replacement, and distributor cap and rotor replacement are all items that need attention before 100,000 miles on most vehicles. I would set a maximum of 3 years or 60,000 miles for a vehicle with an extended tune up interval. Although this may deviate from the manufacturers suggested interval in some cases, engine disassembly because of a stuck spark plug is never what I would consider a “standard” tune up.